1. Field of the Invention
The present invention relates to a novel discharge lamp lighting circuit, which protects a lighting circuit or a discharge lamp from an abnormal operation originated from variations in an input voltage and an input current to be supplied to the lighting circuit from a power supply.
2. Description of the Related Art
Recently, a compact discharge lamp (e.g., a metal halide lamp) is receiving greater attention as a light source which takes the place of an incandescent lamp. It is known that a lighting circuit for such a discharge lamp, as adapted to a light source for a vehicular lamp, includes a DC power supply, a switching power supply circuit, a DC-AC converter, and an igniter circuit.
It is known that variations in the input voltage and input current to a lighting circuit from a power supply may adversely affect the operation of the lighting circuit and the lighting state of a discharge lamp. There is a circuit structure which is designed in view of such phenomenon and which has a predetermined reference value for comparison set for the input voltage or the like and stops power supply to a discharge lamp when the input voltage or the like exceeds or falls below this reference value.
When a short cyclic variation occurs in the input voltage or the input current, however, this circuit causes flickering of a discharge lamp by frequently repeating power supply to the discharge lamp and the inhibition of such power supply. This reduces the visibility of the light from the discharge lamp and may adversely affect the service life of the discharge lamp.
When a sudden drop of the input voltage and/or the input current temporarily occurs, this lighting circuit instantaneously inhibits power supply to a discharge lamp to inevitably turn off the discharge lamp even in the case where the discharge lamp can be kept lit if the input voltage and/or the input current is restored to the proper level immediately after the sudden and temporary drop.